One of the commonly utilized circuits in any electronic device is a voltage divider shown in FIG. 1. An input voltage VIN can be scaled according to the ratio of resistors R1 and R2; one is able to calculate the output voltage of the divider by well-known formula in FIG. 1.
Two familiar applications for the voltage divider are depicted in FIG. 2.
Shown in (a) is the typical circuit for the voltage regulator; the voltage divider consisting of RFB1 and RFB2 generates a feedback voltage VFEEDBACK that is used to regulate the output voltage VOUT. The alert reader will immediately recognize that any faults within the voltage divider will produce an abnormal output voltage from the regulator; this may lead to destruction of the whole device that incorporates this voltage regulator, as well as to smoke and fire.
For example, if resistor RFB2 is opened (due to overheating, failure of the solder, or any other reason), the voltage regulator circuit will assume that the output is low, near 0 V, and will try to increase it uncontrollably.
Therefore, it is typical that any circuit that may produce dangerous output in case of the divider fault includes some redundancy or an independent control mechanism to limit the maximum output. A characteristic solution for the voltage regulator is to utilize two voltage dividers, with the circuit inside of the voltage regulator being able to select the highest feedback voltage, and thus limit the output; alternatively, a separate over-voltage protection circuit is employed.
Illustrated in FIG. 2 (b) is a so-called Instrumentation Amplifier (IA), a circuit that is able to produce at the output an amplified difference between the two input signals.
An important characteristic of the IA is the Common Mode Rejection Ratio (CMRR), an ability to reject changes in input voltages that are common to both inputs, while the difference between the inputs gets amplified and goes though unimpeded.
With the common-day state-of-the-art components it is possible to create an IA circuit of this configuration that boasts a CMRR value of, perhaps, 20-48 dB (utilizing the 0.1% accurate resistors, that are the best available for practical use). Any further improvement of CMRR is achieved by manually trimming the value of one or several resistors.
However, even a perfectly adjusted IA will tend to lose the CMRR value when operated over some temperature range, and specifically when operated at a temperature other than at the temperature at which the adjustments were made. This is due to resistors having various temperatures and/or various temperature coefficients. It will be appreciated that the best possible practical performance for this type of the circuit is on the order of 1% or worse. It is only with strict laboratory conditions and very expensive high-accuracy resistors that better performance is achievable.
On the other hand, the IA constructed according to FIG. 2 (b) is able safely to sense voltages that are many times higher than the supply voltage of the Operational Amplifier, which is a very desirable property.
In a typical system, it is likely that an Analog-to-digital converter will follow the IA, and the IA output will thus be converted to a digital value. Also it is likely that in a modern-day system a microcontroller will make sense of and act upon the values received from the IA.
Furthermore, present-day analog-to-digital converters can have an accuracy and resolution that is many times better than the best-possible performance available from the IA circuit in FIG. 2 (b).